// Package process runs child commands for ReleaseKit's internal adapters.
package process

import (
	"context"
	"errors"
	"fmt"
	"io"
	"os"
	"os/exec"
	"sort"
	"strings"
	"sync"
)

// Invocation describes one child process. UnsetEnv removes inherited values
// before Env overlays values for this child only.
type Invocation struct {
	Args     []string
	Dir      string
	Env      map[string]string
	UnsetEnv []string
	Stdin    io.Reader
	Stdout   io.Writer
	Stderr   io.Writer
}

// Run starts the command in the requested directory and waits for it to exit.
// Errors preserve the underlying exec error, including *exec.ExitError.
func Run(ctx context.Context, invocation Invocation) error {
	command, err := newCommand(ctx, invocation)
	if err != nil {
		return err
	}
	command.Stdin = invocation.Stdin
	command.Stdout = invocation.Stdout
	command.Stderr = invocation.Stderr

	if err := command.Run(); err != nil {
		if contextErr := ctx.Err(); contextErr != nil {
			return fmt.Errorf("run %q: %w", invocation.Args[0], contextErr)
		}
		return fmt.Errorf("run %q: %w", invocation.Args[0], err)
	}
	return nil
}

// PipedProcess owns the command started by StartPiped. Callers close Stdin,
// read Stdout to EOF, and call Wait once after a normal exchange. On an abort,
// close active pipes, call Cancel, and then Wait to reap the child.
type PipedProcess struct {
	command  *exec.Cmd
	stdin    io.WriteCloser
	stdout   io.ReadCloser
	parent   context.Context
	cancel   context.CancelFunc
	waitOnce sync.Once
	waitErr  error
}

// Stdin returns the child process input pipe. The caller owns its closure.
func (child *PipedProcess) Stdin() io.WriteCloser {
	return child.stdin
}

// Stdout returns the child process output pipe. The caller must consume it
// before Wait on a normal exit and owns its closure on an abort.
func (child *PipedProcess) Stdout() io.ReadCloser {
	return child.stdout
}

// Cancel interrupts the child without cancelling its parent context.
func (child *PipedProcess) Cancel() {
	child.cancel()
}

// Wait waits for and reaps the child. It preserves the parent's cancellation
// cause and otherwise returns the command's wrapped exit error.
func (child *PipedProcess) Wait() error {
	child.waitOnce.Do(func() {
		err := child.command.Wait()
		child.cancel()
		if err == nil {
			return
		}
		if contextErr := child.parent.Err(); contextErr != nil {
			child.waitErr = fmt.Errorf("run %q: %w", child.command.Args[0], contextErr)
			return
		}
		child.waitErr = fmt.Errorf("run %q: %w", child.command.Args[0], err)
	})
	return child.waitErr
}

// StartPiped starts a child with stdin and stdout connected to pipes. Command
// settings come from invocation; its Stdin and Stdout fields are ignored, while
// Stderr remains routed as requested.
func StartPiped(ctx context.Context, invocation Invocation) (*PipedProcess, error) {
	childContext, cancel := context.WithCancel(ctx)
	command, err := newCommand(childContext, invocation)
	if err != nil {
		cancel()
		return nil, err
	}
	command.Stderr = invocation.Stderr
	stdin, err := command.StdinPipe()
	if err != nil {
		cancel()
		return nil, fmt.Errorf("pipe stdin for %q: %w", invocation.Args[0], err)
	}
	stdout, err := command.StdoutPipe()
	if err != nil {
		_ = stdin.Close()
		cancel()
		return nil, fmt.Errorf("pipe stdout for %q: %w", invocation.Args[0], err)
	}
	if err := command.Start(); err != nil {
		_ = stdin.Close()
		_ = stdout.Close()
		cancel()
		return nil, fmt.Errorf("start %q: %w", invocation.Args[0], err)
	}
	return &PipedProcess{
		command: command,
		stdin:   stdin,
		stdout:  stdout,
		parent:  ctx,
		cancel:  cancel,
	}, nil
}

func newCommand(ctx context.Context, invocation Invocation) (*exec.Cmd, error) {
	if len(invocation.Args) == 0 || invocation.Args[0] == "" {
		return nil, errors.New("process invocation requires a command")
	}
	command := exec.CommandContext(ctx, invocation.Args[0], invocation.Args[1:]...)
	command.Dir = invocation.Dir
	if len(invocation.Env) != 0 || len(invocation.UnsetEnv) != 0 {
		command.Env = environmentEntries(invocation.Env, invocation.UnsetEnv)
	}
	return command, nil
}

func environmentEntries(overlay map[string]string, unset []string) []string {
	environment := make(map[string]string, len(os.Environ())+len(overlay))
	for _, entry := range os.Environ() {
		key, value, ok := strings.Cut(entry, "=")
		if ok {
			environment[key] = value
		}
	}
	for _, key := range unset {
		delete(environment, key)
	}
	for key, value := range overlay {
		environment[key] = value
	}

	keys := make([]string, 0, len(environment))
	for key := range environment {
		keys = append(keys, key)
	}
	sort.Strings(keys)

	entries := make([]string, 0, len(keys))
	for _, key := range keys {
		entries = append(entries, key+"="+environment[key])
	}
	return entries
}